At a Glance

Why Get Tested?

When to Get Tested?

When you have symptoms suggestive of a carcinoid tumor such as flushing, diarrhea, and/or wheezing

Sample Required?

A blood sample drawn from a vein in your arm

Test Preparation Needed?

None

The Test Sample

What is being tested?

This test measures the amount of serotonin in the blood. Serotonin is a chemical derived from the amino acid tryptophan. It is produced as needed by the nervous system, mainly the brain, but also by special cells in the bronchial tubes (lungs) and gastrointestinal tract. More than 90% of serotonin in the blood is found in the platelets. Serotonin helps transmit nerve impulses and constrict blood vessels, is a participant in the wake-sleep cycle, and affects mood. Serotonin is metabolized by the liver and its metabolites, primarily 5-HIAA (5-hydroxyindoleacetic acid, a muscle stimulant), are excreted in the urine.

Normally, serotonin is present in small varying quantities in the blood. Large quantities of serotonin and 5-HIAA may be produced continuously or intermittently by some carcinoid tumors. Carcinoid tumors are slow-growing masses that can form in the gastrointestinal tract, especially in the appendix, and in the lungs, although they may affect other organs as well. They are one of several types of tumors that arise from cells in the neuroendocrine system - cells that are found in organs throughout the body and that have both nerve and endocrine aspects. The serotonin produced by carcinoid tumors may cause symptoms such as flushing of the face, diarrhea, a rapid heart rate, and wheezing, especially when the tumor has spread to the liver. This group of symptoms is referred to as the carcinoid syndrome.

According to the American Cancer Society, there are about 11,000 to 12,000 neuroendocrine tumors or cancers diagnosed each year in the United States. Many more of these tumors may exist, but most remain small and do not cause any symptoms. When carcinoid tumors are discovered in asymptomatic patients during surgical procedures performed for other reasons, they are called "incidental" tumors. A small percentage of these tumors may eventually grow large enough to cause obstructions in the intestines or bronchial tubes of the lungs.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm.

Is any test preparation needed to ensure the quality of the sample?

The Test

How is it used?

Serotonin may be ordered along with, or following, a 24-hour urine 5-HIAA test to help diagnose carcinoid tumors. It is not generally used as a monitoring tool to evaluate the effectiveness of treatment or to detect recurrence of a carcinoid tumor. Monitoring tests may include 5-HIAA and Chromogranin A.

What does the test result mean?

A significantly increased level of serotonin in a person with carcinoid syndrome symptoms is suggestive but not diagnostic of a carcinoid tumor. In order to diagnose the condition, the tumor itself must be located and biopsied. The doctor will frequently follow an abnormal test result with an order for an imaging scan to help locate any tumor(s) that may be present.

Someone with symptoms may still have a carcinoid tumor even if the concentrations of serotonin and 5-HIAA are normal. The affected person may have a tumor that does not secrete serotonin or one that secretes it intermittently. A person with no symptoms and normal or low levels of serotonin and 5-HIAA is unlikely to have a serotonin-secreting carcinoid tumor.

Is there anything else I should know?

There are a variety of drugs that can affect the serotonin test, including morphine, monoamine oxidase (MAO) inhibitors such as reserpine, methyldopa, and lithium. People should talk to their doctor before decreasing or discontinuing any medications.

Serotonin concentrations may be slightly increased in those with intestinal obstructions, acute myocardial infarction (heart attack), cystic fibrosis, and dumping syndrome. The serotonin test is not usually ordered with these conditions.

Common Questions

1. Should I have both the serotonin and 5-HIAA tests performed?

Serotonin and 5-HIAA offer complementary information. In some cases, 5-HIAA is preferred because it is more stable and, since it is collected for 24 hours, there is more chance of detecting increased 5-HIAA than in identifying excess serotonin that is only released intermittently. Talk to your doctor about which tests are appropriate for your condition.

2. Are some people at a higher risk for developing a carcinoid tumor?

Anyone at any age can develop a carcinoid tumor but, according to the American Cancer Society, the average age at diagnosis is usually about 55 to 65. People with a family history of multiple endocrine neoplasia (MEN1), a genetic condition that increases a person's risk of developing tumors in the endocrine system glands, may be at higher risk for developing a carcinoid tumor.

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Article Sources

NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

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This article was last reviewed on May 10, 2011. | This article was last modified on January 27, 2014.

The review date indicates when the article was last reviewed from beginning to end to ensure that it reflects the most current science. A review may not require any modifications to the article, so the two dates may not always agree.

The modified date indicates that one or more changes were made to the article. Such changes may or may not result from a full review of the article, so the two dates may not always agree.